TISSUE FORMATION 301 



While in certain respects amoebocytes and free-living protozoa differ from 

 each other in their behavior as far as amoeboid movement is concerned, there 

 are also some important similarities in these cell types ; to mention only one 

 feature common to both : the primary and principal change in the consistency 

 of the protoplasm occurs especially at the point where the pseudopod forma- 

 tion takes place, which is the leading and most active and sensitive part of the 

 cell. Connected presumably in some way with the characteristics of the pseu- 

 dopods are their fine reactions to individual and species differences, which 

 have been observed in certain protozoa and which we have already discussed. 

 These reactions also depend on changes in the consistency of the protoplasm, 

 especially of the surface of the cells, which take place in accordance with the 

 degree of compatibility or lack of compatibility between the cells which meet ; 

 and as we have seen, similar changes are also the principal factors leading to 

 pseudopod formation. 



There are, however, also some important differences between amoebocytes 

 and protozoa. In the case of amoebocytes, their behavior, and in particular the 

 degenerative processes they undergo, vary greatly in different media and 

 under different physical conditions. Characteristic of these cells also is their 

 need of a protein medium. The free-living protozoa, on the other hand, are 

 adapted to a medium free of protein. Associated with this difference in the 

 protein requirement of these organisms there is a further difference in their 

 reaction towards certain ions. 



As to the possible role substances corresponding to organismal differentials 

 play in the behavior of amoebocytes, there are individual variations observed 

 in the reactions of the cells and consequently also of the amoebocyte tissue 

 derived from different Limuli. Such variations are as a rule, however, mani- 

 festations of the quantitatively different tendency on the part of amoebocytes 

 to contract and of associated differences in the consistency of these cells; 

 these result mainly from environmental conditions to which the Limuli have 

 been previously subjected. There seems to be no difference in the behavior of 

 amoebocytes to one another, homoiogenous and autogenous amoebocytes 

 behaving in the same way. Therefore there is no manifestation of an indi- 

 viduality differential or a similar substance noticeable in these cells, as far 

 as their mutual reactions are concerned. They differ in this respect from the 

 protozoa, which we have discussed in the preceding chapter. 



The behavior of amoebocytes and the agglutination process leading to a 

 joining together of cells have been considered somewhat more in detail because, 

 as stated, the analysis of experimental amoebocyte tissue shows clearly the 

 principles underlying tissue formation in general, and the union of cells in 

 tissues is the basis of the formation of multicellular organisms. But our con- 

 clusions apply only to the granular amoebocytes, such as those of Limulus. 

 The so-called hyaline amoebocytes which have been studied in recent years, 

 especially by Faure-Fremiet, behave somewhat differently and do not lend 

 themselves to experiments with tissue formation in the same way as the 

 amoebocytes of Limulus. 



In certain respects related to tissue formation is the process which leads 



